MARKET MILK

Module 6. Common dairy operations

Lesson 29
HEAT EXCHANGERS - PLATE AND TUBULAR TYPE

29.1 Introduction

The plate heat exchanger has found wide application in pasteurization and sterilization. It consists of a series of plates, terminals between the plates and a head terminal on to which the plates are pressed with the end terminal. For installation, cleaning and changing of plate rubbers, the plates and intermediate terminals can easily be moved backwards and forwards on carrying bars in a frame. Liquids can be passed in and out of the plant via the intermediate, head and end terminals. The liquid can flow alternately with a colder or warmer medium through the plates in such a manner that one plate occurs in zones close to walls because of low rates of flow.

Fig. 29.1 Plate Heat exchanger for short time heating of milk

29.2 Design and Operating Principle of a Plate Heat Exchanger

The configuration of the individual sections of a plate heat exchanger is shown in the figure (Fig.29.1). Several heat exchange plates are assembled and are installed section wise in a frame. In many cases, plate heat exchangers for short-time heating have no holding section, but they have a tubular holding pipe besides the plate heat exchanger. This makes it possible to keep plate heat exchanger very small and space efficient. Ultra-high heat treatment plate heat exchangers are designed in a modular form.

29.3 Heat Exchanger Plates

Heat exchanger plates (Fig. 29.2) are made from stainless steel and have a heating surface of 0.2-0.4 m². A relatively large surface, considering the overall dimensions, is achieved by the fishbone-like pressed surface pattern. At the same time, very good turbulence is achieved between the plates (Fig. 29.3), thus creating nearly identical heat transfer conditions for all product particles, and a different thermal load is excluded. Close to the passage openings and between the distances, holders are shaped into the plates, which maintain a uniform distance between the plates. Additional longitudinal-shaped cams in the area of the inlet serve for good product distribution over the entire surface of the plate.

Fig. 29.2 Heat exchanger plate

Fig. 29.3 Turbulence between plates

Sealing the flows from each other and from the outside is done on the periphery of the plates and around the inlet/outlet openings with profiled gaskets, which are glued into the correspondingly shaped grooves. Further the plates are set up for hydrodynamic reasons in such a way that the inlet and outlet for the media are on the same side of the plate. This means that the inflow of the medium is on one side and the outflow is on the other side, resulting in simplified piping connections and reduced assembly costs. In each section, wherever there is heat transfer, one medium enters through the inflow and a second exits into the return flow.

In order to have a uniform fluid velocity between the plates and fluid distribution over the entire plate surface, the fluid velocity of the media must be 20 – 25 m/s. This fluid velocity depends among other things, on the flow rate and pressure drop. As the pressure drop decreases with longer distance, considerable pressure differences can be observed between the inflow and the outflow.

29.4 Connecting Plates

They are installed between the plate assemblies of the individual sections and separate them from each other. Connecting points, which can be replaced in most cases, permit the connection to other installations (Fig. 29.4 and 29.5).

Fig. 29.4 Connecting plate

Fig. 29.5 Connecting corner

29.4.1 Flow patterns and circuits

A flow is formed between two plates of a plate assembly. By sequencing the various plates, flow patterns and stages are established in a section. Parallel flow patterns form one stage. The stages are installed in series in a section. The number of parallel flows per stage and the serial stages are chosen according to circumstances; the heating and holding section should not be modified by the operator. A simple flow pattern is shown in figure (Fig. 29.6).

29.5 Heating in a Plate Heat Exchanger

Heating is done with either steam or hot water, which is made by using steam. Constant pressure and temperature conditions in the steam supply pipes are the basis for a non interrupted process. The choice of a correctly calculated pipe diameter of the steam pipe lines and effective pressure control are the basis for constant conditions. For heating purposes, saturated steam (no superheated steam) is used, which should have the quality of a wet steam (2-8% water content). Pressure should be 0.96¬ to 1.96 bar.

Fig. 29.6 Heat exchange and flow directions in heat exchanger

a) Milk inlet, right side b) Milk outlet, right side c) Steam inlet, left side d) Condensate outlet, left side e) Water inlet, left side f) Water outlet left top 01, F, G and so on indicate the plates, 1...4 and 5 ... 9 the number of plates.

Fig. 29.7 Heat supply to a plate heat exchanger for short time heat treatment

Heat supply to a plate heat exchanger for short-time heat treatment

Fig. 29.8 Heat supply to a plate heat exchanger for short-time heat treatment

1. Steam distribution 2. Mixing battery 3. Plate heat exchanger

Steam injection is used in plate heat exchangers both for the high-heat process and for heating cream. Due to the relatively wide differences between the steam temperature and the product temperature as well as the high steam temperatures (> 100°C), the risk of burning milk is high and therefore enhanced depositions in the heating section must be considered (Fig 29.7). Short-time heating and thermization use hot water, which can be obtained in three different ways:

● In a separate section of the plate heat exchanger

● In a mixing nozzle

● In a hot water battery

The flow in figure (Fig. 29.8) shows that the pressure-reduced steam passes through a membrane valve into the hot water mixing battery, where it is mixed with water. The membrane valve controls the steam quantity as a function of the set temperature and the valve is actuated pneumatically, adjusting the hot water temperature accordingly. The steam distributor can be eliminated if wet steam can be sup¬plied directly from the steam boiler, as wet steam cools down along its way in the steam pipes. Steam is dried by pressure reduction; it does not superheat.

29.6 Tubular Heat Exchangers

In tubular heat exchangers, milk products flow through tubes which are heated or cooled externally (Fig 29.9 and 29.10). The flow can be more easily controlled in the case of single tube than in a bundle of tubes. The advantage of a bundle of tubes is that larger heat exchange surface can be fitted economically into a smaller space. The spirally arranged single tube tubular heater, consisting of coaxial double tubes saves space and can be used for sterilizing milk. The milk can pass through the following stages in a continuous stream:

Preheating by means of the return flow of milk through the annular space. Heating with hot water or steam. Holding at pasteurizing or sterilizing temperature. Cooling by giving up heat to the incoming milk. Cooling with chilled water or brine.

A separator is often inserted between the preheater and the heater and a homogenizer before or after the heater.

Fig. 29.9 Tubular heat exchanger

The fact that tubular heat exchanger can only be cleaned by flow through methods (usually 1%, HNO3 and 1% NaOH,) is a disadvantage. Deposits on the heating surfaces cannot be seen and are hardly removable by mechanical means.

Fig. 29.10 Heating plant with tubular heater

With this type of heat exchanger it is, however, possible to work at high pressures which are of advantage when high temperatures are to be used. Sealing problems do not arise.